cytosine methylation stability

Anyone have any idea how stable cytosine methylation in vivo is when subjected to environmental insults? What product it converts to over time? I assume it deaminates into thymidine? Any idea how long that it takes under which conditions?

DNA methylation is a stable and inheritable epigenetic marker. Though there are studies that exposure to certain environmental substances can cause aberrant hypermethylation, there is no much data on how fast DNA methylation changes in response to insults.

You are right, 5' methyl cytosines are not stable, they may deaminate into thymidine, which account for some mutation found in DNA.